The present invention relates to a hydraulic chain or belt tensioner having a tapered or stepped geometry in either the bore or the piston of the tensioner.
Hydraulic tensioners are typically used as control devices for chain or belt drives in an automotive engine timing system. Such chain or belt drives provide a driving connection from the crankshaft to the camshafts and to auxiliary devices, such as an oil or fuel pump. During typical operation, the tension in the chain or belt can vary greatly due to the dynamic shaft oscillations which are induced from engine firing, valve train loading, or auxiliary device operation. Moreover, chain or belt components typically wear during prolonged use thereby increasing the length of the chain or belt and decreasing the tension on the chain or belt. A hydraulic tensioner is used to take up the slack in the chain or belt and provide control of the chain or belt oscillations due to tension fluctuations.
A typical hydraulic tensioner of the prior art, as shown in FIG. 1, is comprised of a housing 1 having a cylindrical bore 2, and a hollow cylindrical piston 3 biased in a protruding direction from the bore 2 by a spring 4. Also included in the hydraulic tensioner is a check valve comprised of housing 5, spring 6, and check ball 7. The check ball 7 is large enough to close off oil supply passage 8. The check valve allows pressurized fluid to pass through passage 8 into the fluid chamber 9 (defined by hollow piston 3 and bore 2) through the check valve seat 10 and through holes in the check valve housing 5 while preventing flow in the reverse direction.
During typical operation of the tensioner, the piston 3 encounters forces not only from the chain and spring 4, but also fluid forces in the fluid chamber 9. When the chain is slack, the net force acts to move the piston axially outward of the bore, and thereby decreases the chamber fluid pressure to a value less than that of the supply pressure. This pressure drop allows pressurized fluid in the oil passage 8 to push the check ball 7 away from the seat 10 and allows oil to flow into the chamber 9. When the chain tightens, the net force acts to move the piston axially inward of the bore, resulting in an increase of pressure in chamber 9. The increase in pressure subsequently pushes the check ball 7 onto the seat 10, closing off the oil passage 8. The force of the chain inward on the piston is opposed by the fluid pressure in the chamber and the spring force. Any subsequent inward travel of the piston is thus dictated by the amount of oil which can escape or leak through the remaining clearance space 11 between the piston and bore.
The clearance space 11 and the size of the passage for leakage of oil are dictated by the orientation of the piston itself relative to the bore. For a cylindrical piston and bore, the centerline of the piston and bore are not necessarily parallel or coincident during operation, nor is there any mechanical means available to provide bore-piston concentricity. As a result, the orientation of the piston relative to the bore can become misaligned or non-concentric during operation, allowing for axial misalignment and radial motion of the piston relative to the bore. In particular, if the piston axis is at a maximum radial eccentricity relative to the bore, the oil flow may be up to 2.5 times greater than that obtained when both the piston and bore are concentric. This causes uncertainty in oil leakage and also causes uncertainties in chamber pressure and piston motion. Moreover, the possibility of relatively large eccentric piston positioning may result in higher friction, stiction, and increased wear.
Suzuki et al., in U.S. Pat. No. 5,352,159, address the problem of piston leakage. Suzuki et al. show a tensioner with a piston slidable within a cylinder that is divided into a low pressure chamber and high pressure chamber. A seal surrounds the upper end of the piston to prevent oil leakage, and a check valve allows oil flow from the low pressure to the high pressure chamber. An oil reservoir supplies oil to the low pressure chamber and has an opening to collect oil from oil mist within the engine and surrounding the tensioner. When the engine is non-operational for an extended period of time, the Suzuki et al. tensioner reservoir may leak and allow oil to drain from the fluid chamber and reservoir. If the oil drains out of the reservoir, the piston will draw air into the chamber at start-up. This causes excessive travel of the piston and noise during start-up and low pressure idle conditions. In addition, the Suzuki et al. tensioner is expensive to manufacture because of the cost of manufacturing each part of the tensioner separately.
An additional problem encountered by tensioners during operation is that, during leakage of oil around the piston, air may become trapped within the piston chamber. The air prevents the piston chamber from filling properly. Therefore, it is desirable to vent the air trapped in the piston chamber. A hydraulic tensioner that allows air to vent is shown in Todd, U.S. Pat. No. 5,370,584. A piston with a tapered base directs air upward to the vent, or gap, formed between the piston and piston cavity. Preferably, the tapered base is a tapered cone having a base to height ratio of 3:1.
As explained in Viersma, T. J., Analysis, Synthesis, and Design of Hydraulic Servosystems and Pipelines, Elsevier, 1980, electro-hydraulic servo cylinders and actuators, stepped and tapered pistons have been employed to reduce Coulomb friction and avoid stiction or momentary contact of the center piston rod and bore. The problem of stiction occurs when the piston rod is not kept concentric with the bore and catches on the side of the bore. Stepped and tapered rods have also been applied to machine tools, hydraulic presses, and flight simulators.
The problem of stiction between the piston and bore is decreased in the tensioner of the present invention by including tapers or steps in either the piston itself or the bore. According to the theory presented by Viersma, whenever the oil pressure in a tapered piston is greater on the side of the piston with the larger clearance than on the side that converges and provides a smaller clearance, film pressures are generated in the clearance area which in turn produce radial forces on the piston. If the piston is eccentric, the gap pressure or pressure at the point between the piston and the wall will be greater at points where the gap is the smallest, producing radial forces which will tend to align the bore and piston axes. As a result, friction is reduced for the larger oil films present, and the resulting oil flow through the gap can be better controlled.